The present invention relates to bimodal rubbers and rubber modified monovinylidene aromatic polymers produced therefrom.
A variety of rubbers have been used in producing rubber modified monovinylidene aromatic polymers. It is well known in the art that the balance of gloss, impact and rigidity properties of such rubber modified polymers are dependent on rubber particle size, particle size distribution, rubber level and flow properties. Typically, polymers comprising small rubber particles have high gloss, high rigidity and low impact properties, while polymers comprising large rubber particles have low gloss, low rigidity and high impact properties.
Standard rubbers, commonly known as linear butadiene homopolymer types, cannot attain the small particles sizes needed to obtain high gloss products. Block copolymers usually lead to small rubber particles, but are expensive. Star branched rubbers have been produced by anionic polymerization processes in order to achieve a high degree of coupling, as is described in EP-277,687. EP-277,687 also discloses a rubber modified polymer containing radial or branched polybutadiene rubber particles having a volume average diameter of 0.1 to 1.2 microns (.mu.) and rubber particles containing either radial, branched or linear rubber having a volume average particle diameter of from 1 to 5.mu.. However, while these compositions have high gloss, they do not have sufficient toughness and rigidity. EP-418,042 discloses a rubber modified monovinylidene aromatic polymer using radial or star branched rubber polymers, which have been partially coupled, and have a bimodal weight average molecular weight (Mw) distribution. Generally a rubber having a bimodal Mw distribution will typically produce rubber modified products having a bimodal particle size or a broad particle size distribution. Rubber modified resins produced using such rubbers generally have good gloss, tensile yield strength and izod impact balance. However, they also have lower Gardner impact properties and lower overall elongation properties. Additionally, rubber modified polymers having a narrow rubber particle size distribution have low gloss and tensile yield strength properties.
Therefore, it is highly desirable to produce rubber modified polymers using rubbers having a broad, yet monomodal rubber particle size distribution, while having improved tensile yield strength. Such polymers have the combined properties of increased practical toughness as indicated by Gardner impact and also rigidity as indicated by tensile yield strength.
Thus, there remains a need to produce a rubber having a broad yet monomodal Mw, which leads to a broad, yet monomodal rubber particle size distribution, with the desirable properties of good practical toughness and rigidity.